Implantable, self-expanding prosthetic device

ABSTRACT

A prosthetic device for sustaining a vessel or hollow organ lumen (a stent) has a tubular wire frame ( 1 ) with rows of elongate cells ( 2 ) having a larger axis and a smaller axis. The cells are arranged with the larger axis in the circumferential direction of the frame ( 2 ) and the smaller axis parallel to the axial direction thereof. Each cell is formed by two U-shaped wire sections, and in a plane perpendicular to the longitudinal axis one of the branches of the U-shaped wire sections in one row form together a closed ring shape ( 4 ) which provides the frame ( 1 ) with large radial stiffness. In the axial direction the frame ( 1 ) has only low stiffness so that it easily conforms to the vascular wall even if this deforms due to external loads. The interconnection between the cells ( 2 ) may be flexible.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of copending U.S. DesignPatent Application Ser. No. 29/034,346, filed Feb. 2, 1995, entitled “AnImplantable, Self-Expanding Stent” and commonly assigned herewith, whichapplication is a continuation-in-part of U.S. Utility Patent ApplicationSer. No. 08/379,582, filed Feb. 1, 1995, entitled “A Prosthetic Devicefor Sustaining a Blood Vessel or Hollow Organ Lumen,” which applicationis the U.S. national phase of International Patent Application No.PCT/DK93/00256, filed Aug. 6, 1993, which application claims priority toRussian Application No. 5057852, filed Aug. 6, 1992 (now Reg. No.35-13-426, granted Feb. 18, 1993).

TECHNICAL FIELD

The invention pertains to implantable medical devices and, inparticular, to a self-expanding prosthetic device for sustaining avessel or hollow organ lumen.

BACKGROUND OF THE INVENTION

Various diseases of blood vessels or hollow organs cause a stenosis orcomplete obturation (occlusion) of their lumen, which results in adecrease or complete loss of their functional attributes. The widespread of diseases of this kind demands an elaboration of quite newmethods of medical treatment.

Devices for sustaining a blood vessel or hollow organ lumen typicallyhave a tubular shaped frame body which is introduced in the vessel orhollow organ and fixed in the necessary place to sustain its lumen. Theproblem of designing such devices has already a twenty year history.Nevertheless, a universally reliable device satisfying all necessaryrequirements has as yet not been created.

A device for sustaining a vessel or hollow organ lumen should satisfythe following requirements:

-   -   effectively fulfill the function of recovering and sustaining        the vessel or hollow organ lumen;    -   have a reliable and simple delivery control system;    -   have a wide range of sizes from 3 to 50 mm and more;    -   have biological compatibility with organism tissues;    -   be useable in different anatomical areas of vessels and hollow        organs;    -   cause minimum trauma during and after operation; and    -   have a stiff construction to counteract in situ external        compression forces.

An attempt to create a device compatible with tissues was undertaken inUSSR Patent No. 1237201, dated Feb. 15, 1986. This known device forsustaining the vessel or hollow organ lumen represents a wire framehaving a tubular shaped body. The frame is formed by a wire element,having round or square cross-section and arranged in a cylindricalhelical line. The frame has a shape of a helical cylindrical spring andit is furnished with fixing elements to keep it on the device fordelivery into the vessel or hollow organ. Each fixing element is made inthe form of a loop, one of which is formed at the initial section of thewire element, and the other at its final section. The facility fordelivering the above device to a vessel or hollow organ comprises anintroducer in the form of an X-ray contrast tube and another X-raycontrast tube of a lesser diameter, on whose surface the device issecured by means of a connecting element. The material of the frame wireis an alloy of the titanium-nickel system, which is biologicallycompatible with the organism tissues.

The device known from the USSR patent is reliable in use. However, it isexpedient to use the known device in vessels or hollow organs having adiameter of not more than 8 mm, which is conditioned by the value of theultimate strain of the frame material limited by 8% (the so-calledstrain limit of shape memory effect), as well as by the demand ofminimizing the puncture hole (hole in vessel through which the device isintroduced into an organ). Furthermore, the device can withstand onlylimited external compression forces.

The use of the known device in vessels and hollow organs with a diameterexceeding 8 mm, and without exceeding the ultimate strain of the framematerial, would demand a decrease of the thickness of the wire frameelements, which would result in a further loss of stiffness of theframe. Alternatively, it would be necessary to increase the diameter ofthe puncture hole, which in turn would cause intolerable trauma to thevascular or hollow organ walls. Thus, the mentioned construction of thedevice for sustaining a vessel or hollow organ lumen is applicable onlyfor vessels or hollow organs whose diameter is less than 8 mm, whichsharply narrows the field of its application.

The execution of the function of effectively recovering and sustaining avessel or hollow organ lumen by the described device demands anarrangement of the coils of the wire frame with a minimum lead toprevent germination of atherosclerotic patches, or counteract theocclusion. However, the making of the frame with a minimum lead betweencoils results in a loss of its stiffness in the vessel or hollow organ.As a result, external compression forces effect a change of the frame'sarrangement in the vessel, i.e., the frame's longitudinal axis getsarranged at an angle to the vessel axis, or in an increase of the leadbetween coils. Both in the first and second cases, the frame stopsfunctioning, and the vessel or hollow organ lumen gets reduced.

As it was described above, the frame is furnished with fixing elementson the front and rear ends. The fixing elements are made in the form ofloops lying in the plane perpendicular to the frame axis in such amanner that the partial overlapping of the frame lumen occurs. As aresult, turbulent flows in the blood current are formed and facilitatethe appearance of various complications such as atheroscleroticformations.

The described facility of frame delivery is reliable enough in theprocess of introduction of the frame to the affected area. However, atinstallation of the frame with the aid of this facility one of thefixing loops gets released. The frame, being scragged up until thismoment, gets released and uncoils in the direction opposite to thedirection of coiling at its fixing, acquiring its initial shape. In theprocess of uncoiling, which is uncontrolled, trauma to the vascular orhollow organ walls may occur, which has an unfavorable affect on theresult of operation. In addition, the frame can occupy an arbitraryposition in the vessel that is uncontrolled by the surgeon.

The described frame has the shape of a helical cylindrical spring. If weexamine the frame section in a plane perpendicular to the frame axis andpassing through the coil surface, it is seen that the frame coil locatedin the plane has a break, which decreases the frame stiffness under theeffect of radially acting forces.

Another device for sustaining a vessel or hollow organ lumen is known(Ann Radiol, 1988, 31, n. 2, 100-103), and it has a tubular shaped wireframe formed by a wire element, which in development represents asaw-tooth line. In order to permit a change in the stiffness of theframe, the latter is bound at the tops by a caprone thread.

The branches of the wire element are arranged along the longitudinalaxis of the tubular frame, which provides for a constancy of the frame'slinear dimensions at the delivery and installation of the frame in theaffected place of the vessel or hollow organ. To fix the frame in thevascular or hollow organ walls, provision is made for fixing elements inthe form of hooks.

In the described construction, use is made of materials whose ultimateelastic strain makes up tenths of a percent. The delivery systemrepresents an X-ray contrast tube accommodating a pusher, which is apiston with a rod. For transportation (delivery), the device is placedin the X-ray contrast tube, and by means of the rod the surgeon actsupon the piston interacting with the device.

The described device has found a wide application for sustaining thelumen of the affected areas of veins, in which there are noatherosclerotic processes. The use of this device in arterial vessels ishardly possible because of the large distances between the wireelements, which may result in germination of atherosclerotic patchesand, as a consequence, in an ineffective use of this device.

The latter known device is used for sustaining the lumen of the affectedareas of veins whose diameter is within 15 to 30 mm. In this case, awire of a large diameter is used to impart the necessary stiffness tothe construction. If this device were to be used in smaller vessels orhollow organs having a diameter from 3 to 15 mm it would be necessary todecrease the wire thickness (diameter). However, the loss in diameterthickness may hardly provide an effective means for sustaining thelumen.

Due to the arrangement of the wire branches in the peripheral directionof the tubular frame body, the given construction is stable and has ahigh stiffness in the axial direction, which prevents full adjustment ofthe vessel geometry and may traumatize the vascular or hollow organwalls.

When it is necessary to deliver the above device to the affected areaalong a curved path, the elastic deformation of the frame wire elementschanges into plastic deformation, which results in an irreversiblechange of the device shape. Thus, delivery of the given frame to theaffected place is possible only along a path close to a straight line,which considerably narrows the number of the anatomical areas, where theframe could be used.

A device of the initially mentioned kind is known from EP-A-221570. Inthis device, the larger axis of each cell is arranged in the axialdirection of the tubular body and the smaller axis in thecircumferential direction thereof. The wire sections forming the cellsare rigidly interconnected.

The delivery facility of the described device comprises an X-raycontrast tube with an inflatable balloon, on the outside of which thewire frame is located. To press the wire frame onto the X-ray contrasttube, provision is made for one more tube enveloping the frame on itsexternal surface. In delivery of the frame to the affected area of thevessel or hollow organ, the external tube is removed, and the balloon isinflated so that the frame is expanded and acquires its final shapewhereafter it interacts with the vascular walls. Then, the X-raycontrast tube is removed from the vessel, and the frame is installed inthe affected area.

Its delivery and installation in the affected area is sufficientlyreliable and convenient. However, the use of a rigid joint by fusingtogether, soldering or welding of the wire elements in the points oftheir intersection seems to be unreliable because of:

-   -   a probable proceeding of electrochemical processes in the        soldering zone, which may cause damage to the joint, loss of        stiffness in the frame and consequently, loss of its functional        attributes; and    -   formation of the so-called welding zone with an embrittled        material structure, which may make this joint unreliable.

The described device can be used for sustaining the lumen of vessels orhollow organs within a range of sizes from 3 to 8 mm. In the describedconstruction use is made of materials whose ultimate elastic strainmakes up tenths of a percent. When it is necessary to deliver the deviceto the affected area along a curved path, a danger arises to exceed theultimate elastic strain and, consequently, the proceeding of the processof plastic deformation of the frame material. Thus, the delivery of thegiven frame is possible only along a path close to a straight line,which essentially decreases the possibility of its use in differentanatomic areas. The known device has a large stiffness in the axialdirection which may traumatize the walls of the vascular or hollow organin the regions around the ends of the device if the device supports avascular or hollow organ which changes its shape during adaptation tovarying external loads. Further, it is a common disadvantage of theknown devices that they possess limited radial stiffness, which allowsthem to support only vascular or hollow organs that are not surroundedby a bone structure taking up external loads.

SUMMARY OF THE INVENTION

The invention is based on the problem of creating a device forsustaining a vessel or hollow organ lumen, in which the shape andarrangement of cells forming the tubular frame provide the frame with alarge stiffness in the radial direction and only low stiffness in theaxial direction so that the device without risk of traumatization willkeep the vascular or hollow organ open, even if the latter changes shapedue to external loads.

This is obtained by a prosthetic device having a wire frame in the formof a flexible tubular shaped body which in development is formed by manyrows of interconnected cells, each of which cells comprises two U-shapedwire sections forming together approximately an elongated oval with alarger axis and a smaller axis, wherein adjacent cells in neighboringrows are shifted by half of the larger axis of the oval with respect toeach other in the direction of the larger axis and are shifted by thesmaller axis of the oval with respect to each other in the direction ofthe smaller axis. According to the invention, the device ischaracterized in that the larger axis of the oval is directed in thecircumferential direction of the tubular body and the smaller axisparallel to the axial direction thereof so that in a plane perpendicularto the longitudinal axis of the body one of the branches of the U-shapedwire sections in one row forms together a closed ring shape.

By arranging the cells so that the larger axis of the oval is directedin the circumferential direction the device has on one hand a largeflexibility in the axial direction which allows the device to bendsimultaneously with the vascular or hollow organ even if the bending isvery localized because the long branches of the U-shape are easilydeformed in the axial direction. In addition, the device is very rigidtowards localized radial compression because the U-shaped branches ofeach row of cells form two circumferential rings having a relativelyhigh stiffness in their plane. The flexibility of the device in theaxial direction further ensures that a local deformation of the vesseldoes not cause the device to lengthen in the axial direction as thedeformation is absorbed within the pressure affected rows of cells. Thiscauses the device to stay fixed with respect to the surroundingsupported wall of the vascular or hollow organ so that traumatization isavoided. Under the action of external compression force, the ring shapeis essentially uniformly loaded. The axial stiffness of the device canto some extent be adjusted as needed by varying the cross-sectional areaof the frame wire. By varying the number of cells in the frame, itbecomes possible to select the optimum axial stiffness of the frame, sothat the vascular or hollow organ wall is traumatized as little aspossible.

In a preferred embodiment, adjacent cells in one row are interconnectedin a flexible manner at the axially extending portion of the U-shapedwire sections. The flexible interconnection allows large deformations ofthe initially unloaded cell geometry without large deformations in thewire proper because the wire sections are not rigidly fixed to eachother.

When the device is to be introduced, the ends of the tubular frame arepulled away from each other and the frame diameter is reduced until theframe can be inserted into a delivery catheter. During lengthening ofthe frame, the major portion of cell deformation occurs in the longbranches of the wire sections, and it is assumed that the axiallyextending portion of the U-shaped wire sections is only slightlydeformed so that the entire U-shaped wire section is substantiallyuniformly loaded. Consequently, the diameter of the tubular frame may bedrastically reduced during insertion without exceeding the elasticstrain limit of the wire material. This makes it possible to use devicesaccording to the invention within a wide range of sizes and to introducethe devices through a small puncture hole in the patient, even if thewire is made of, e.g., stainless steel.

Preferably the flexible interconnections are accomplished by winding theaxially extending portions around each other, more preferably so thatthe one wire portion is wound only one turn around the associated wireportion. During deformation of the U-shaped wire sections, the windingsmay move apart and/or open which reduces strain in the wire. The woundwire portions also act as a kind of hinge joint allowing the twoU-shaped wire sections in a cell to swivel with respect to each otherwhen the frame is radially loaded. The wound flexible interconnectionspresent a further advantage, namely that as an alternative to axiallylengthening of the frame prior to insertion in the catheter the tubularframe may be twisted about its longitudinal axis by turning the twoframe ends in opposite directions. This causes the woundinterconnections to open and the frame to collapse to a reduced diameterallowing insertion. When the frame after positioning abreast of the siteto be supported is pushed out of the catheter it “uncoils” to itsinitial diameter without any substantial axial shortening of the frame,which leads to an uncomplicated and very precise positioning of thedevice in the vascular or hollow organ.

In a further embodiment, which is preferred due to its simplicity ofmanufacture, the device is characterized in that each U-shaped wiresection is composed of two separate wires each of which runs helicallythrough the rows of cells, and that the two wires are wound, preferably,one turn around each other at the axially extending portion where theymeet to form the bottom leg of the U-shape.

The device may have wires of a shape memory alloy exhibiting thermallyactivated shape memory properties, preferably a nickel-titanium alloy,but more preferably the wires are of a shape memory alloy exhibitingsuperelastic properties, advantageously a nickel-titanium alloy. Such ashape memory alloy can be excessively deformed and yet return to its setpredetermined shape without loss of stiffness or introduction ofpermanent deformations in the wire. The shape memory alloy wire framecan be reduced to a diameter of only a few mm during insertionirrespective of its unloaded diameter which, e.g., may be as large as 50mm so that the frame can be introduced into the patient through a singlesmall diameter catheter requiring only a small puncture hole in thepatient. The superelastic alloy is preferred in order to avoid thermalcontrol during insertion. When this alloy is deformed it exhibits stressinduced martensite.

The above-described possibilities of variation of the axial and radialstiffness of the frame allow the latter to fulfill the function ofsustaining a vessel or hollow organ lumen within any range of theirstandard sizes, for example, from a diameter of 3 mm to a diameter of 50mm, and be applicable in different anatomical areas of the vessel orhollow organ and even to be introduced along a tortuous path. The devicemay also be used for retention of blood clots as a Vena cava filter.

The aforementioned prosthetic device of the present invention has beendescribed with the flexible interconnections all being wound in either aclockwise or counterclockwise direction. Although well suited for itsintended purpose, the stent with its flexible interconnections all woundin the same direction exhibits a twisting, spiraling, corkscrewing, oruncoiling motion as it is deployed from the end of a delivery catheteror tube. This uncoiling motion was previously described and isundesirable in that plaque or other material formed on the wall of avessel can be dislodged with undesirable trauma occurring to thepatient. By way of example, this trauma could result in the formation ofan embolism and resultant patient death. To minimize, if not eliminatethis undesirable motion, the flexibly interconnected wire segments ofthe stent are selectively wound in opposite directions to effectivelycounterbalance the stent.

In one embodiment of the present invention, the flexibly interconnectedwire segments of the cells in each row are all wound in the samedirection, whereas the wire segments of the cells in an adjacent row areall wound in an opposite direction. This advantageously counterbalancesthe moments formed by the flexibly interconnected wire segments aroundand along an adjacent pair of rows. Furthermore, the winding of the wiresegments in this manner forms a uniformly shaped wall with a minimumwall thickness.

In another embodiment of the present invention, the flexiblyinterconnected wire segments in each cell are wound in oppositedirections to counterbalance the moments formed in the cell. As aresult, the flexibly interconnected wire segments in each cell and rowof the stent are advantageously counterbalanced. Adjacent loops at oneend of the stent as well as fixedly secured, adjacent wire segments atan other end of the stent are also wound in opposite directions tofurther advantageously counterbalance the stent.

BRIEF DESCRIPTION OF THE DRAWING

In the following description, examples of embodiments of the deviceaccording to the invention are described in further detail withreference to the schematic drawings, in which

FIG. 1 shows a perspective view of the device for sustaining a vessel orhollow organ lumen, according to the invention;

FIG. 2 shows in a larger scale, a development of the frame surface;

FIG. 3 is a section after line III-III in FIG. 1;

FIGS. 4 and 4 a illustrate the delivery device with the frame inlongitudinal section and perspective view, respectively;

FIG. 5 shows in development a section of the frame surface in a secondembodiment according to the invention, in large scale;

FIG. 6 depicts a pictorial view of a third embodiment of the presentinvention; and

FIG. 7 depicts a pictorial view of a fourth embodiment of the presentinvention.

DETAILED DESCRIPTION

The device for sustaining the lumen, for example of the femoral artery,accomplished in accordance with the invention, has wire frame 1 in theform of a tubular shaped body such as a hollow cylindrical body.

The cylindrical surface of frame 1 shown in development in FIG. 2 isformed by a large number of interconnected cells 2 formed by twoU-shaped wire sections 3, interconnected by their branches 3 a, 3 b, andforming approximately an oval, whose larger axis is arranged in thecircumferential direction of the body and the smaller axis parallel toits axial direction. Cell 2 of each subsequent row is shifted in thecircumferential direction with respect to cell 2 of the present row by ½of the length of the oval larger axis. Each branch 3 a or 3 b of theU-shaped wire section 3 belongs to two cells 2 in adjacent rows, exceptfor the first and last rows. In a cross-section of frame 1 in a planeperpendicular to its longitudinal axis and passing through the longbranches of the U-shaped wire sections 3 of one row these branches forma closed ring shape 4 which provides the frame with large stiffness in aradial direction and ensures that the cell will only to a very limitedextent be deformed in the axial direction when it is radially loaded.The wire section 3 may have a circular cross-section as seen in FIG. 3.

The wire can be made of a titanium-nickel alloy having shape memoryproperties which may either be thermally or stress activated. When thewire is of such an alloy which may be heavily deformed without permanentdeformation of the wire, the cells 2 of frame 1 can be interconnected bya rigid joint at the tops of the U-shaped wire sections. Alternativelythe U-shaped sections may be flexibly interconnected by small rings,e.g., of thread.

The described device is introduced into the vessel A such as the femoralartery as follows. A delivery device 5 comprises a hollow X-ray contrasttube 6, containing a hollow pusher 7 with a rod 8. The pusher 7 has aninternal space 9 including two stops 10 in the form of cylindricalradially extending pins rigidly connected with a holder 11 arrangedalong the longitudinal axis of rod 8. The distance between the extremepoints of stops 10 essentially corresponds to the inside diameter ofpusher 7. The holder 11 is installed with a possibility of longitudinaldisplacement.

The frame 1 is secured to the stops 10 of holder 11 by means of lugs 12inserted over the stops 10. The holder 11 connected to frame 1, is fixedwith respect to rod 8. The rod 8 is introduced into X-ray contrast tube6 simultaneously with the frame 1 being drawn into contrast tube 5 alongits longitudinal axis. When entering the contrast tube 6 the sections 3forming frame 1 acquire a shape close to a straight line and the framediameter is reduced to a few mm. The forward end of contrast tube 6 isthen, through the puncture hole, brought to the affected area of vesselA. Frame 1 may alternatively be brought into tube 6 by rotating holder11 with respect to the frame end opposite to the end fixed to stops 10so that the frame 1 is collapsed to a small diameter and may be insertedinto tube 6.

When the delivery device 5 is in position in the vessel or hollow organthe surgeon, while acting upon frame 1 through rod 8, withdraws theX-ray contrast tube from the frame so that the wire sections 3 of thedevice fold out to the original tubular shape.

If the wire is a thermally activated shape memory alloy, the bloodtemperature heats the wire and the device acquires its initial shape. Ifthe wire is superelastic, it will simply return to its preset shape whenthe restraining force from tube 6 is removed.

Recovery of the initial frame shape occurs in succession by formingclosed ring-shaped circuits 4 in the plane perpendicular to the deviceaxis. The ring-shaped circuit interacts with the walls of vessel ororgan A (FIG. 4 a), sustaining its lumen constant and repeating itsgeometry due to the maximum radial stiffness and optimum axial stiffnessof frame 1 (FIG. 1). The described constructional features of the devicemake it possible to bring it to the affected area through a minimumpuncture hole.

The embodiment shown in FIG. 5 has cells 2 of similar shape as in theabove described embodiment, but the cells are interconnected in analternative manner. Each U-shaped wire section is composed by twodifferent wires which run in a substantially helical shape or helicallythrough the rows of cells and the wires and are wound one turn aroundeach other at the axially extending wire portion where they meet to formthe bottom portion or leg of the U-shape. At the ends of the frame, theassociated pairs of wires are joined at points B and C by twisting thewires around each other. The formed loop can be bent into the adjacentouter cell in order not to traumatize the vascular wall. The formedinterconnections between the cells are highly flexible, and the wirescan deform more or less independently of each other.

The device is introduced into the hollow vein, artery or organ A in thesame manner as the above described device.

The device, preserving a constant diameter for the vessel lumen andmaintaining or reinstating its geometry, has an increased durabilitybecause of the movable joint between the wires.

The accomplished analysis and the obtained positive estimate of thebiological compatibility of the described device made it possible toperform bench tests. The mechanical characteristics of the device werestudied on a special model of the arterial system of a human being, andthe technical elements of the procedure of its implantation in differentareas of the vascular channel were elaborated.

The bench tests have displayed good qualities of the described deviceand made it possible to conduct experimental investigations on animals.

Experiments were conducted on 10 dogs, 3 of them were subjected to anacute experiment, and 7 were subjected to dynamic observations.Implantations were accomplished into the thoracic, abdominal aortas,renal, iliolumbar and femoral arteries. During X-ray analyses, it wasgenerally noted that the devices did not shift from the places of theirinitial implantation, the device shape conforms to its initial one, andno symptoms of thrombosis or stenosis of the vessel were revealed.

FIG. 6 depicts a pictorial view of another embodiment of the presentinvention. In this embodiment, prosthetic device 20 is an implantable,self-expanding stent and includes a wire frame 1 having a flexibletubular shape 13 and a plurality or, more particularly, rows 25 ofinterconnected cells 2 with flexible interconnections 14. The stent isdepicted in an expanded condition with four flexibly interconnectedcells forming a row and extending around the circumference of the stent.Each cell has first and second substantially U-shaped wire sections 3.Each U-shaped wire section 3 includes first and second flexiblyinterconnected wire segments 15 and 16 each of which runs in astair-step manner helically along the wire frame and through the rows ofthe interconnected cells. The flexibly interconnected wire segments arewound around each other at an axially extending portion 17 of the wireframe and form the flexible interconnections of each cell as well as theU-shaped wire sections of the cells. To specifically address theuncoiling problem of the prosthetic device or stent when the stent isreleased from the delivery tube 6 and into a vessel or hollow organlumen, the flexible interconnections 14 or, more particularly, flexiblyinterconnected wire segments 15 and 16 in each U-shaped wire section ofa cell are wound in opposite, counterclockwise and clockwise directions18 and 19. The oppositely wound flexible interconnections in each cellof a given row counterbalance the twisting moments not only created bythe flexible interconnections in each cell, but also counterbalance thetwisting moments created in the row of interconnected cells.

To further minimize, if not eliminate, the twisting or uncoiling motionof the stent when being released from the delivery tube, each of thefirst and second wire segments in a pair 24 runs in a stair-step mannerhelically along the wire frame and through the interconnected cells.This stair-step helical configuration is also shown in FIG. 5. However,returning to FIG. 6, wire segment 15 runs in a counterclockwise helicaldirection 22, whereas wire segment 16 runs in an opposite, clockwisehelical direction 23. As indicated, first and second wire segments 15and 16 are grouped in pairs 24. As a result, the number of wire segmentsrunning helically in a counterclockwise direction are equivalent innumber to the wire segments running helically in a clockwise direction,thereby counterbalancing each other. To further counterbalance thestent, the pairs of wire segments are also even in number as depicted instent 20 of FIG. 6.

The prosthetic device and, in particular, stent 20 also includes at oneend 32 an even number of loops 28. A pair 24 of first and second wiresegments 15 and 16 extend from each loop. By way of example, a separatepiece of wire about its midpoint is bent around a cylindrical peg toform a loop 28 and a pair 24 of first and second wire segments 15 and16. The wire segments are wound a half turn and then around acylindrical mandrel in a stair-step manner and a helical direction toform the stent. In this example, stent 20 includes four loops of whichtwo opposing loops are wound in counterclockwise direction 18, and theadjacent opposing loops are wound in clockwise direction 19. This isdone to maintain the stent in a counterbalanced condition around itscircumference. At opposite end 33 of the stent, pairs 36 of first andsecond wire segments 34 and 35 are wound together and fixedly secured toeach other with a weld or solder bead 38. Four or five turns are made atend 33 of the stent so that the flexible interconnection of the wiresegments is maintained in the vicinity of the last row of cells.Furthermore, the four or five turns of the wire segments provide abuffer for the flexible interconnection due to the deterioration of thesuperelastic or shape memory property of the wire material when solderor weld beads 38 are formed. Adjacent pairs of fixedly secured wiresegments 34 and 35 are wound in opposite directions 18 and 19.

As previously suggested, first and second flexibly interconnected wiresegments are wound in opposite directions at the laterally extendingportions of each U-shaped cell. As depicted in stent 20 of FIG. 6, allthe flexible interconnections forming a laterally extending column ofthe stent are wound only one turn and alternate in direction everysecond row.

FIG. 7 depicts yet another preferred embodiment of the presentinvention. In this preferred embodiment, prosthetic device or stent 20includes a wire frame 1 having a flexible tubular sheath 13 and rows 25of interconnected cells 2. Each of the cells has first and secondsubstantially U-shaped wire sections 3, wherein each substantiallyU-shaped wire section includes first and second flexibly interconnectedwire segments 15 and 16. Each of the flexibly interconnected wiresegments runs helically along the wire frame in a step-like manner andthrough the interconnected rows of cells. The flexibly interconnectedwire segments and each U-shaped wire section are wound around each otherone turn at axially extending wire portion 17 of the frame. Aspreviously suggested, one or more of the U-shaped wire sections arewound in counterclockwise direction 18, while at least others of thewire sections are wound in a clockwise direction 19 opposite todirection 18. Unlike the cells of stent 20 of FIG. 6, all the U-shapedwire sections in, for example, row 26 of interconnected cells are woundin counterclockwise direction 18. To circumferentially andlongitudinally counterbalance row 26, all the U-shaped wire sections inadjacent row 27 of interconnected cells are wound in clockwise direction19.

To maximize the radial strength of the stent, the long branches ofU-shaped wire sections 3 are formed in a closed ring shape 4, that iscontained in a plane perpendicular to the longitudinal axis of thestent. The wire of the stent is preferably a nickel-titanium alloyhaving shape memory and superelastic properties. Preferably, thetransformation temperature of the nickel-titanium alloy is selected tobe below the normal temperature of a human body, whereby the alloy is inan austenitic state exhibiting its superelastic property. After thestent is formed, typically around a cylindrical mandrel, the fullydeployed tubular shape of the nickel-titanium alloy stent is heat set ina well-known manner at a temperature typically well above itstransformation temperature. Once heat set, the stent wants to return toits fully deployed tubular shape after being, for example, stretched orelongated for insertion in a delivery tube or catheter. However, in thefully deployed tubular shape, the branches of each U-shaped wire sectionare in a plane perpendicular to the longitudinal axis of the stent,which provides maximum radial strength for the stent. Each of flexibleinterconnections 14 is positioned at an axial portion 17 of the wireframe and functions as a flexible hinge when the diameter of the stentis being either expanded or contracted. The flexible hinge is formed bywinding the first and second wire segments only one turn 31 around eachother.

Similar to the previously described embodiments, stent 20 of FIG. 7 alsoincludes at one end 32 a plurality of loops 28 from each of whichextends a pair 24 of flexibly interconnected wire segments 15 and 16.Each loop is formed by winding wire segments 15 and 16 only half a turn37. This maintains the flexible interconnection of the segments as wellas a counterbalance with the flexible interconnections that extendaxially along the length of the stent. In this embodiment, the fiveloops of the stent are all wound in a counterclockwise direction 18.However, adjacent loops, such as 29 and 30, can be wound incounterclockwise and clockwise directions 18 and 19 around thecircumference of the stent. This alternate loop winding direction ispreferred where the loops or pairs of flexible wire segments are even innumber.

At an other end 33 of the stent, flexibly interconnected wire segments34 and 35 are fixedly secured to each other with, for example, awell-known solder or weld bead 38. In this embodiment of the stent, thefixedly secured wire segments are all wound in counterclockwisedirection 18. An even number of pairs can be wound in oppositecounterclockwise and clockwise directions to further facilitatecounterbalancing of the stent.

The flexibly interconnected wire segments in each U-shaped wire sectionof a given row are all wound in the same direction, whereas wiresegments in each U-shaped wire section of an adjacent row are wound inan opposite direction. This provides counterbalancing of the flexiblyinterconnected wire segments longitudinally along pairs of adjacentrows. Although the wire segments are not counterbalanced in each row,the wall thickness of the stent along its length is uniform with a lowprofile, thus contributing to the desirability of this configuration.

It is to be understood that the above-described prosthetic device ismerely an illustrative embodiment of the principles of this inventionand that other devices or stents may be devised by those skilled in theart without departing from the spirit and scope of this invention. Inparticular, it is fully contemplated that the flexible interconnectionsof the stent can be formed with any number thereof in either thecounterclockwise or clockwise directions. At a minimum, one flexibleinterconnection need be wound in a given direction with the remaininginterconnections being wound in the opposite direction. The number offlexible interconnections in opposite directions is only limited by theamount of twisting, spiraling, corkscrewing, or uncoiling of the stentthat is clinically acceptable to the attending physician. However, theamount of twisting, corkscrewing, spiraling or uncoiling motion of thestent should be minimized so as to reduce the risk of shearing orcutting plaque from the wall of, for example, a blood vessel. Suchtwisting, corkscrewing, spiraling, or uncoiling motion of the stent canalso cause trauma to delicate or fragile blood vessels, ducts, and thelike.

1-54. (canceled)
 55. A prosthetic device comprising: a flexible, tubularshaped frame of flexibly interconnected cells, each of the cellsincluding first and second U-shaped sections forming together anapproximately elongated oval with a larger axis and a smaller axis,wherein the larger axis of the oval is directed in a circumferentialdirection of the tubular shaped frame and the smaller axis is directedin a direction parallel to a longitudinal axis of the tubular shapedframe, and wherein in a plane perpendicular to the longitudinal axis ofthe tubular frame, one branch of each U-shaped section in one rowtogether form a closed ring shape.
 56. The prosthetic device of claim 55wherein frame comprises a superelastic material.
 57. The prostheticdevice of claim 56 wherein the superelastic material comprises anickel-titanium alloy.
 58. The prosthetic device of claim 55 wherein theflexible, tubular shaped frame is self-expanding.
 59. The prostheticdevice of claim 55 wherein adjacent cells in neighboring rows areshifted by a predetermined amount of the larger axis of the oval withrespect to each other in the direction of the larger axis.
 60. Theprosthetic device of claim 59 wherein adjacent cells in alternating rowsare shifted by the smaller axis of the oval with respect to each otherin the direction of the smaller axis.
 61. A prosthetic devicecomprising: a frame having a flexible tubular shaped body with rows ofinterconnected cells, the flexible tubular shaped body having a reduceddiameter during introduction with a tube or a delivery catheter, each ofthe cells having two U-shaped sections forming an approximatelyelongated oval with a larger axis and a smaller axis, wherein adjacentcells in neighboring rows are shifted by half of the larger axis of theoval with respect to each other in the direction of the larger axis,wherein the larger axis of the oval is directed in the circumferentialdirection of the tubular body and the smaller axis parallel to the axialdirection thereof so that in a plane perpendicular to the longitudinalaxis of the body, one branch of each U-shaped section in one row formstogether a closed ring shape, and wherein adjacent cells in one row areinterconnected in a flexible manner at an axially extending portion ofthe U-shaped sections.
 62. A prosthetic device of claim 61 wherein theframe comprises a shape memory alloy exhibiting thermally activatedshape memory properties.
 63. A prosthetic device of claim 62 wherein theshape memory alloy comprises a nickel-titanium alloy.
 64. A prostheticdevice of claim 62 wherein the shape memory alloy exhibits superelasticproperties.